Well you don't need any special code. Just upload a picture into an album you have on your profile. Once you've uploaded the picture you want as your background, copy that URL and put it in the "Main" section for profile customization where it says "Background Image".
You have to go to your profile customization page (you can get there by clicking "My Account"). There you can pick your colors and background pictures.
It looks like you've started customizing your profile already. Is there anything in particular you are confused about?
You get pictures on the internet and post them onto an album. The album has to be labeled "profile" (there are three options: public, private, and profile) and then you go to the edit profile and there should be a picture option somewhere...ok, this might be a bit confusing to follow...